I picked up a Bushnell 3-9x40mm from Gander Mountain for $70 and a pair of weaver mounts for another $20 and everything fits.
I tested it out last sat. on the farm and was able to get 6 rounds within a dime of center at 60 yards. Only took 3 rounds to sight it in! (maybe I am just that good...not!)
It is my first AR (well...any firearm larger than a 22LR) and my first "high power" scope.
My AR is a DPMS semiauto with flat top (low riser) and 20" barrel with floating grip and bipod.
Best present I ever bought myself. <hehe>
Most Bushnells I've used were decent, if not good scopes. It should last a while on an AR.